Multiple editable field should be real multiple

[peter-gribanov]

Subject

Multiple editable field should be real multiple.

Now a multiple field is shown as multiple but edit is not a multiple and new value will be overridden, not append.

Closes #5767

Changelog

### Added
- Multiple editable fields now is a real multiple.

[greg0ire]

Please kindly squash your commits together. If you don't, we'll try to remember to do it for you but it's best if you save us this trouble.

How to do that?

1. git rebase -i origin/3.x, assuming origin is a git remote that points to this repository, and not your fork. If you're not sure what your remotes are, run git remote -vvv, there should be your fork and the holy/reference/base/origin/whatever-you-call-it repository.
2. A window will show up with many lines, replace pick with fixup on every line but the first one
3. Close your editor, git should do its magic, and you should end up with one commit
4. Use git push --force to overwrite what you already push. Don't forget the --force option otherwise git will try to merge both things together.

Also, please format your commit message according to our rules :

• The commit message subject must be less than 50 characters and tell us what you did.
• The commit message body should tell us why you did it. It is optional but highly recommended.

Bad example :

Fixed bug #989 by removing call to defraculate()

Good example:

Remove call to defraculate()

Calling this function caused a bug because it interferes 
with calls to getSchmeckles().
Fixes #989

More details in CONTRIBUTING.md

[peter-gribanov]

@greg0ire i squash commits

[greg0ire]

Thanks @peter-gribanov !